1. Field of the Invention
The present invention relates to an oxide superconductor having a relatively high superconductivity transition temperature T.sub.c and a method for producing it.
2. Description of Related Art
YBa.sub.2 Cu.sub.3 O.sub.7-z (0.ltoreq.z.ltoreq.1.0) has been known as an oxide superconductor having a superconductivity transition temperature (T.sub.c =90K). It has also been known that substitution for a part of Y of other rare earth element(s) R causes substantially no change of T.sub.c.
However, in order that the superconductors RBa.sub.2 Cu.sub.3 O.sub.7-z can have a high T.sub.c, the process for the production thereof must be controlled so that the oxygen loss Z is close to zero (0). For this purpose, heat treatment for a long time of, for example, 5-10 hours at a temperature of 400.degree.-450.degree. C. has been generally needed after firing materials at a temperature of 900.degree.-950.degree. C. in an oxygen gas and annealing them at a cooling rate of 1.degree.-2.degree. C./min. ["OYOBUTURI", Vol. 57 (1988), pp. 1217-1218]. Such heat treatment has complicated the process for the production of the above-mentioned type superconductors and this has caused increase in cost for the production of wires or thin films therefrom.
Hitherto, for obtaining the effect as of the present invention, it has been attempted to substitute Ca at the Y site of YBa.sub.2 Cu.sub.3 O.sub.7-z. Kosuge, B. Okai, K. Takahashi and M. Ohta, "Jpn. J. Appl. Phys.", 27 (1988), pp. L1022-L1024]. However, since the amount of Ca to form a solid solution with Y, namely, the amount (x) of Ca substituted in (Y.sub.1-x Ca.sub.x)Ba.sub.2 Cu.sub.3 O.sub.7-z is small (0.1 or less), no sufficient effect has been obtained. [F. Devaux, A. Manthiram and J. B. Goodenough, "Physical Review B", Vol. 41 (1990), pp. 8723-8731].
Furthermore, it has been known to substitute a rare earth element at the Y site of YBa.sub.2 Cu.sub.3 O.sub.7-z. [J. M. Tarascon, W. R. McKinnon, L. H. Greene, G. W. Hull and E. M. Vogel, "Physical Review B", Vol. 36 (1987), pp. 226-233].
Moreover, it has also been known to substitute Sr at the Ba site of YBa.sub.2 Cu.sub.3 O.sub.7-z. [T. Wada, S. Adachi, T. Mihara and R. Inaba, "Jpn. J. Appl. Phys.", 26 (1987), pp. L706-L-708].